The present invention relates to a thermoresistance adhesive, a thermoresistance adhesive solution, a thermoresistance resin paste, a process for the preparation thereof, a semiconductor chip having a thermoresistance adhesive layer, a lead frame having a thermoresistance adhesive layer, a film having a thermoresistance adhesive layer, and a semiconductor device.
The resin-sealed semiconductor devices are typically of the structures such as illustrated in FIGS. 1 to 3. A thermoresistance adhesive is used for bonding the inner leads of a lead frame to a semiconductor chip in the semiconductor device of FIG. 1, or bonding the inner leads of a lead frame which doubles as a buffer coat to the chip surface in the semiconductor device of FIG. 2. In an ordinary semiconductor device shown in FIG. 3, the said adhesive is utilized for bonding a lead frame tab (die pad) to the back side of a chip. The structures of the semiconductor devices of the type illustrated in FIGS. 1 and 2 are called lead-on-chip (LOC) structure, and the thermoresistance adhesive used therefor is required to be capable of providing secure bonding (especially heat bonding) of the chip and lead frame and to also have enough adhesiveness to the sealer to prevent package cracking in solder reflowing.
Recently, with advancement of miniaturization of semiconductor devices, the proportion of the chips in the semiconductor devices has increased while the rate of the sealer used in such devices has decreased, and this situation is blamed for the frequent occurrence of package cracking, a phenomenon caused as the moisture absorbed in the thermoresistance adhesive or sealer is vaporized and expanded by the heat such as generated from soldering treatment (solder reflowing). In order to prevent such a phenomenon, it has been attempted to lower hygroscopicity or elevate glass transition temperature of the thermoresistance adhesive and to split the adhesive mass into a plurality of small pieces, thereby allowing escape of the water vapors in solder reflowing to prevent cracking (JP-A 3-109757). With the conventional adhesives, however, it has been hardly possible to provide both secure bonding (especially heat bonding) of the chip and lead frame and enough adhesiveness to the sealer to prevent package cracking in solder reflowing.
The heat-resistant resins such as polyimide resins have already been widely used for surface protective films, interlaminar insulating films, etc., of semiconductor elements in the field of electronics as these resins have excellent mechanical properties as well as high heat resistance. Recently, as means for forming an image on such polyimide films, attention is focussed on screen printing which can dispense with such steps as exposure, development and etching. A thixotropic heat-resistant resin paste composed of a filler, a binder and a solvent is used for screen printing. In most of the hitherto developed thermoresistance resin pastes, fine silica particles or non-soluble fine polyimide particles are used as the filler for affording the thixotropic properties, so that these resin pastes involve the problem that many voids or air cells are formed at the filler interface during heat drying to lower the film strength. There have been developed the thermoresistance resin pastes (such as the one disclosed in JP-A 2-289646) which are free of such problems and capable of forming a high-quality polyimide pattern by using a combination of an organic filler (soluble filler), a binder and a solvent, in which in the course of heat drying, the filler is first dissolved and then compatibilized with the binder to form a film. In production of this type of resin paste, mechanical milling such as roll milling is required as means for mixing and dispersing the fine silica particles or non-soluble polyimide particles and a specific organic filler (soluble filler) in a binder/solvent solution. According to this method, however, dust or other ionic impurities tend to mix in the thermoresistance resin paste from the mixer and/or the mixing atmosphere, so that this technique was unsuited for such uses as production of semiconductor elements and also unsatisfactory in terms of productivity. Further, since the specific organic filler (soluble filler) is generally produced by a reprecipitation method in which a dilute polyimide resin solution is supplied into a poor solvent of the polyimide resin and the precipitated fine solid particles are recovered, the process was complicated and low in productivity.
An object of the present invention is to solve these problems and to provide a thermoresistance adhesive which is capable of providing secure bonding (especially heat bonding) of a chip and a lead frame and also has sufficient adhesiveness to the sealer to prevent package cracking in solder reflowing, a solution of such a thermoresistance adhesive, and a semiconductor chip having a thermoresistance adhesive layer, lead frame having a thermoresistance adhesive layer, a film having a thermoresistance adhesive layer, and a semiconductor device.
Another object of the present invention is to provide a thermoresistance resin paste which is capable of affording the thixotropic properties to said elements with no need of using a filler such as fine silica particles or non-soluble polyimide particles, and also makes it possible to form a pattern uniform in thickness, high in reliability and free of voids or air cells by screen printing, and a process for producing such a thermoresistance resin paste containing few contaminants such as dust or ionic impurities with high productivity.
Thus, the present invention provides a thermoresistance adhesive to be used for bonding a semiconductor chip and a lead frame in a resin-sealed type semiconductor device, characterized in that the said adhesive does not dissolve in the sealer composing resins at the sealer molding temperature, and that its semiconductor chip/lead frame adhesive strength under shear is 1 N/4 mm2 or greater.
The present invention further provides a thermoresistance adhesive solution containing an organic solvent in addition to the said components of the said thermoresistance adhesive.
It is also envisaged in this invention to provide a thermoresistance resin paste comprising (A) a heat-resistant resin having a hydroxyl group, an amino group or a carboxyl group in the molecule, (B) fine organic particles, (C) a crosslinking agent having functional groups chemically bondable to the said hydroxyl, amino or carboxyl group, and (D) a solvent, characterized in that before heat drying, the fine organic particles (B) exist as a heterogeneous phase as opposed to the homogeneous phase composed of the heat-resistant resin (A), crosslinking agent (C) and solvent (D), and after heat drying, there is formed a homogeneous phase containing the heat-resistant resin (A), fine organic particles (B) and crosslinking agent (C) as essential components.
The present invention further provides a thermoresistance adhesive obtained by drying either the said thermoresistance adhesive solution or the said thermoresistance resin paste.
The present invention also provides a semiconductor chip having a thermoresistance adhesive layer produced by providing a layer of said thermoresistance adhesive on the circuit forming side of a semiconductor chip.
The present invention further provides a lead frame having a thermoresistance adhesive layer produced by providing a layer of said thermoresistance adhesive on the semiconductor chip-mounted side of a lead frame.
The present invention further provides a film having a thermoresistance adhesive layer produced by providing a layer of said thermoresistance adhesive on one or both sides of a support film.
The present invention is also intended to provide a semiconductor device in which the plural inner leads of a lead frame are bonded to the circuit-forming side of a semiconductor chip by the said thermoresistance adhesive, and the semiconductor chip and the inner leads of the lead frame are electrically connected by wire bonding, said semiconductor chip being sealed by a sealant.
The present invention further provides a process for producing a thermoresistance resin paste which comprises mixing (I) a heat-resistant resin A soluble in the solvent of (III) at room temperature and at the heat drying temperature, (II) a heat-resistant resin B which is insoluble in the solvent of (III) at room temperature but soluble at the heat drying temperature, and (III) a solvent, dissolving the said materials by heating, and cooling the obtained soluble to have the fine particles of the heat-resistant resin B of (II) precipitated and dispersed in the solution of the heat-resistant resin A of (I) and the solvent of (III).